Verifications of analytical and strength prediction methods for bolted composite joints are generally based on experimentally obtained data from double-lap joint specimens. The joint or test material having a hole is sandwiched between two load transfer or clevis plates which are clamped together with a through bolt. Joint failure modes and failure stresses in net tension, bearing, and shearout are obtained with varying joint geometry.
The most important joint parameters are the ratios of test specimen width/test specimen hole diameter, distance from the center of test specimen hole to edge of test specimen/test specimen hole diameter, and test specimen thickness/test specimen hole diameter. The clamping force applied to the joint by applying torque to the bolt is also important. Joint load carrying capability and failure mode are determined from load-displacement curve(s) and visual inspection of the test specimen hole. Joint displacement is measured with displacement transducers or other extensometer devices attached to the joint near the bolted hole. In mechanically fastened joints the stresses are at a maximum at the fastener hole.
The ability to accurately measure hole elongation without affecting the joint parameters would provide a better understanding of the elastic and plastic behavior of the joint material leading to failure. Determining the operable failure mechanisms in mechanically fastened joints is needed for the design of more efficient, lightweight composite joints.
Various techniques have been employed to experimentally measure hole elongation at a mechanically fastened composite joint. The double-lap test joint is the generally used method to obtain experimental data to support the validity of analytical models which predict stress levels and failure modes of a loaded hole. For a double-lap test joint, a displacement device or reference attachment is mounted to the outer load transfer plates and another reference attachment is mounted to the joint or test material. By mounting one reference attachment to the load transfer plates, the measured displacement is a combination of deformations in the test material and deformations in the load transfer plates. Thus, an accurate determination of the deformations occurring in the test material is dependent on the accuracy of the prediction and calculation of stresses and deformations in the load transfer plates.
Accordingly, it is an object of the present invention to provide a device which will subject a test specimen having a hole to a tensile loading and measure the resulting hole elongation.
A further object is to provide a device which will determine the operable failure mode of mechanically fastened lightweight composite joints by measuring the hole elongation of a bolted joint.
It is another object of the invention to provide a device which will obviate long standing problems in obtaining specimen deformation measurements.